The present invention relates generally to an optical system that converges the light from a light source onto a target, and more particularly to an immersion optical system that fills a liquid between a final lens surface and a target and an optical apparatus having the immersion optical system.
For an improved resolution of an optical system, such as a microscope and a semiconductor exposure apparatus, the immersion lithography is used which fills an immersion material having a refractive index greater than 1 in a space between a target and a final lens surface of an optical system and improves an effective numerical aperture (“NA”). The final lens surface of the optical system for the immersion lithography is often designed to a plane for filling easiness of the immersion material in the space.
FIG. 18 shows an optical system that uses the conventional immersion lithography. The optical system includes a final lens 201 and an immersion material 202. The light is incident upon a lens 201, passes through the immersion material 202, and is converged at a maximum incident angle θi onto a target 203. The optical system has an NA below, where ni is a refractive index of the immersion material:NA=ni×sin θi
Resolution R and depth of focus DOF are expressed as follows, where λ is a wavelength, k1 and k2 are proportionality factors that depend upon a process and an illumination condition:R =k1×λ/NADOF=k2×λ/[2×ni×{1−(1−(NA/ni)2)1/2}]
Hence, as the NA is made larger, a value of the resolution R reduces and provides a fine optical image.
The larger NA increases the depth of focus DOF, and mitigates a deterioration of an optical image due to a target position error and a focus position error.
Where air having a refractive index of 1.0 is filled in a space between the lens and the target, and the maximum incident angle in the air is θ0, at which the light is incident upon the target, the maximum angle of θ0 is 90°, the maximum value of NA is 1, and it is impossible to reduce the resolution R. However, when the immersion material having a refractive index ni greater than 1 is filled between a space between the lens and the target, NA can be made greater than 1, and the depth of focus DOF can be increased irrespective of a smaller resolution (higher resolving power).
When the optical system uses a flat final surface, a spherical aberration occurs between an existence and a non-existence of the immersion material due to a thickness of the immersion material. In order to reduce this aberration, one proposed technology defines a radius of curvature of a final surface of an immersion projection optical system in a semiconductor exposure apparatus. See, for example, Japanese Patent Application, Publication No. 2000-58436, paragraph nos. 0009-0011, FIG. 2, etc.
FIG. 19 is a view around a final lens in a conventional immersion projection optical system. As illustrated, the immersion projection optical system includes a final lens 201, an immersion material 202, and a tank 204 that stores the immersion material 202. The light 200 is incident upon the lens 201, and passes the immersion material 202, and is converged at a maximum incident angle θi onto the target 203. The lens 201 has a concave spherical, final surface having a center at one point on the target 203. When this shape is used and the light is incident perpendicularly upon an interface between the final surface of the lens 201 and the immersion material 202, the light is condensed on the target 203 without refractions irrespective of a refractive index difference. This configuration prevents a spherical aberration that would otherwise occur in case of a plane-parallel plate, even when the immersion material is removed.
However, as shown in FIG. 18, when the final lens surface is plane, a maximum value NA0 of NA is determined by a refractive index ni of the immersion material and the maximum incident angle θi in the immersion material under condition of ni0<nl, where n1 is a refractive index of a glass material of the lens, and ni0 is a refractive index of the immersion material, as follows:NA0=ni0×sin θi
While the theoretical maximum value of sin θi is 1, the refractive index on the final lens surface actually suddenly increases as it becomes close to 1, and NA cannot be increased up to the refractive index of the immersion material.
In addition, in an attempt to use the immersion material having a refractive index ni higher than a refractive index of a glass material nl for an increased NA, the maximum value NA1 of NA is determined by the refractive index n1 of the glass material and the maximum incident angle θ1 upon the glass material as follows:NA1=nl×sin θl
This configuration can increase NA greater than ni0 but not greater than the refractive index nl of the glass material, because of a relationship ni0<nl<ni and the theoretical maximum value sin θi of 1. In other words, even in an attempt to use the immersion material having a refractive index greater than that of the lens glass material, the rays having large incident angles in the light incident upon the final lens surface are totally reflected on the final lens surface and never reach the target. Therefore, the NA of the optical system cannot be increased up to the refractive index of the glass material. Moreover, the NA cannot be theoretically increased greater than the refractive index of the glass material.
FIG. 20 shows an NA dependency of the light intensity reflectance on the final lens surface or a maximum incident angle dependency of the light, where the refractive index nl of the glass material is 1.50, the refractive index ni of the immersion material is 1.63, and the incident light is a p-polarized light. FIG. 21 also shows an NA dependency of the reflectance where the incident light is an s-polarized light. From these figures, when the NA approaches to the refractive index nl of the glass material of 1.50, e.g., 16.5% of the p-polarized light and 22.4% of the s-polarized light are reflected in the light having the maximum incident angle when NA=1.48, and the imaging characteristic suddenly deteriorates. Thus, the NA cannot be increased up to the refractive index of the glass material. In addition, in an attempt to increase NA up to the refractive index of the glass material, the light having the maximum incident angle is entirely reflected, and it is thus impossible to make NA of the optical system equal to and greater than the refractive index of the glass material.
One solution for this problem that NA of the optical system cannot be increased up to the refractive index of the glass material with a plane final lens surface is to provide the final lens surface with a shape having a radius of curvature corresponding to the NA. Nevertheless, a structure that defines the radius of curvature of the final lens surface, as in the above prior art reference, does not weigh the spread of an area in which the light is irradiated onto the target, and cannot form a good optical image on an entire surface of the light irradiation area, although a good optical image can be obtained at one focal position proportional to λ/NA.